US3183554A - Molding apparatus - Google Patents

Description

May 18 l965 G. R. BRANDT MOLDING APPARATUS Original Filed Oct. 31, 1961 ifi N R M5. @A N@ w w N United States Patent O 3,183,554 MOLDlNG APPARATUS 'Glendon R. Brandt, White Bear Lake, Minn., assigner to Minnesota Mining and Manufacturing Company, St. Paul, Minn., a corporation of Delaware Original application Oct. 31, 1961, Ser. No. 148,974. Divided and this application Apr. 9, 1963, Ser. No.

3 Claims. (Cl. 18-26) This application is a division of my prior and cepending application Serial No. 148,974, filed October 31, 1961.

This invention relates to apparatus for molding high viscosity ilumine-containing polymers. Another aspect of this invention relates to apparatus for preparation of selfsupporting films of fluorine-containing polymers.

Numerous uorine-containing thermoplastic and elastomeric polymers are known in the art. The homopolymer of trifluorochloroethylene is a thermoplastic having a high molding temperature and a very high viscosity under molding conditions. Other fluorine-containing polymers of a similar nature include thermoplastic and elastomeric homopolymers of triuorochloroethylene or tetraiiuoroethylene and copolymers of trifluorochloroethylene or tetrauoroethylene with other ethylene-unsaturated liuorine-containing monomers such as vinylidene fluoride, vinyl fluoride, peruoropropene, triuoropropene, tritiuorobutadiene and ditiuorodichloroethylene. Self-supporting films have been prepared from these thermoplastic and elastomeric polymers. However, the technique for preparing self-supporting films from these polymers is complicated and expensive. The most common method for preparing films is extrusion. However, the films thus produced frequently contain iiaws due to bubbles and in many instances are of non-uniform thickness. In addition, the conditions under which the extruded lilm is made causes severe degradation of the polymer which results in a film having properties less desirable than the original polymer. Some of these deficiencies of extruded film may be overcome by casting films from solutions or plastisols. However, most of the ilumine-containing polymers are not soluble in conventional solvents and plastisols are difiicult if not impossible to make from the polymers. The few solvents that may be used are very expensive and hazardous.

One specific form of film is an endless belt. Such endless belts have many conventional uses known to those skilled in the art. However, it has been substantially impossible to produce a satisfactory endless belt of liuorinecontaining polymers and when it is possible, the process technique, which is usually extrusion, is expensive and wasteful. The extruded tubing must be cut to size. The resulting endless belt has the same deficiencies as the above-described films and in addition is under considerable stress because of the orientation of the polymers during extrusion. As a result of the stress or orientation, the dimensions of the endless belt change upon ageing.

It is much to be desired, therefore, to provide apparatus for producing plastic films from ilumine-containing polymers which overcome the above difficulties.

An object of this invention is to provide apparatus for making self-supporting lilms of fluorine-containing polymers.

Still another object of this invention is to provide a more economical molding apparatus which eliminate proeessing waste and trimming operations.

Still another object of this invention is to provide apparatus for making a film which is substantially free from bubbles and other similar flaws.

Various other objects and advantages of the present invention will become apparent to those skilled in the art from the accompanying description and disclosure.

3,l83,554 Patented May 18, 1965 F ice According to this invention, films or other molded articles are prepared from ilumine-containing polymers by uniformly spreading powdered polymer inside of an axially rotating molding cylinder. The powdered polymer is uniformly spread throughout the molding cylinder while the cylinder is being rotated at a sufficiently high speed that the polymer particles are stationary. Due to the fact that ilumine-containing polymers have such a high viscosity at the molding conditions, it is impossible to introduce the polymer into the molding cylinder in a fluid condition, and it is impossible for the polymer to become sufficiently fluid at the elevated temperature of .molding to uniformly distribute itself upon the rotating cylinder surface. Therefore, it is essential to the present invention that means be provided for uniformly and evenly distributing the powdered particles upon the inside surface of the rotating cylinder.

After the powder is thus uniformly distributed in the rotating molding cylinder, the molding cylinder is heated under confine/.i conditions to a sntiiciently high temperature while still rotating that the powdered particles fuse together in the form of a continuous, self-supporting film. This condition is achieved with the use of elevated ternperatures for suflicient time for incipient fusion to take place.

After the powdered polymer has achieved its state of fusion sucient to form a continuous surface, preferably homogeneous, on the inside of the rotating cylinder, the rotating cylinder is then rapidly cooled while still rotating to solidify the film within the cylinder. Cooling is usually achieved by spraying or contacting the cylinder with water or other cooling fluid. After the rotating cylinder is cooled, the rotating cylinder is then stopped for the first time, and the film is peeled from the inside of the molding cylinder in the form of an endless belt or film. The resulting film is substantially free from bubbles and other flaws, is dimensionally stable because it is substantially free from orientation and is of uniform gauge and thickness. The inner surface of the belt or film will correspond to the surface of the rotating molding cylinder; therefore, it is essential when it is desired to obtain a smooth film to use a smooth molding surface. On the other hand, where congurations or designs are desired on one Asurface of the film, the molding cylinder may be cut or grooved corresponding to the desired design. The film will reproduce the design precisely. Thus a knurled belt can be easily and conveniently produced in this manner.

The uniformity of the distribution of powdered polymer upon the molding surface of the rotating drum determines the uniformity of the gauge thickness of the resultant iilm. Molding under confined conditions assures a smooth surface on both sides of the film because it prevents the evaporation of plasticizers from the polymer and decreases the` turbulence of the air within the molding cylinder. Uniform heating of the molding cylinder is also essential to provide an ultimate lm of uniform physical properties.

FIGURE l of the accompanying drawing is a top view of a diagrammatic illustration of the molding apparatus used in the process of this invention.

FIGURE 2 isa broken-away, enlarged cross-section of the molding cylinder.

FIGURE 3 is a diagrammatic view in cross-section of the feeding mechanism, viewed along 3-3 of FIGURE l.

FIGURE 4 shows a diagrammatic elevational end view of the cooling apparatus, viewed along 4-4 of FIG- URE 1.

For a better understanding of the invention, reference will now be made to the drawing with a description of the parts of the apparatus and their operation in accordance with this invention.

v conventional means.

3 FIGURE 1, as previously stated, is the top view of the apparatus. Numeral 19 indicates a cylindrical molding drum which is in a position ready for molding. The feeding position of molding cylinder 19 is shown in relief with relation to a feeding mechanism indicated by numeral 7. The cooling position of the molding drum 19 (shown in relief) is indicated by numeral 47. The molding drum 19 is positioned in each of these positions by being pivoted on a pivot plate 30 which supports a driving mechanism 28 for cylinder 19.

Proceeding with the various operating stages in sequence, the feeding mechanism 7 comprises a feeding trough 8 of triangular cross-section as shownin cross'- sectional view along line 3--3 in FIGURE 3. Trough 8 as shown in FIGURE 3 has an angular side 9 upon which is positioned a sliding or adjustable side 11 to regulate the opening of elongated slot 12. The positioning of -adjustable side 11 is effected by conventional means not shown. Opening 12 is regulated in relation to the particle i size of the molding powder such that the distribution of the powdered polymer within the cylindrical drum 19 is uniformly effected. Feeding trough 8 is spaced above a slidable base 13 and is attached to base 13 through arm 16 by conventional means not shown. Base 13 slides in a track 15 so that it may be moved backward and forward as shown in the drawing to be inserted or removed from the inside of molding cylinder 19. When inserting trough 8, slidable base 13 moves under and free from cylinder 19. Numeral 14 is a conventional electrical or pneumatic vibrator which is attached to the fixed end of trough 8 by means of arm or bar 16. The vibrator facilitates the uniform ow of particles through slot 12'of feeding trough 8.

The molding apparatus is shown by numeral. 18 in FIGURE 1. This apparatus includes the cylindrical moldn ing drum or cylinder 19, motor 28 and oven 31. The

inside molding surface of cylinder 19 is indicated by numeral 20. This surface is smooth when it is desired to prepare a smooth film, or designed or grooved when powder therein and for engaging a circular end plate 33.

FiGURE 2 shows an enlargedcross-section of lip 22. The inside angled portion of lip 22 provides for channeling small size plastic particles back onto molding surface 20. Molding drum 19 is attached by a rotatable shaft 24 to -a conventional motor 28. Shaft 24 passes through a bearing 26 which is attached to a stationary circular end plate 27, which end plate is attached to pivot plate 30 by means not shown. Shaft 24 passes through end plate 27 and is freely rotatable in bearing 26. End plate 27 serves as a closure for an oven 31, to be discussed hereinafter, and as a support for rotatable shaft 24. Motor 28 is firmly affixed to pivot plate 30 which can revolve 180 or more, so as toy place molding cylinder 19 into position for loading with powder from feeding trough 8 or for cooling in cooling apparatus 47. Rotation of pivot plate 30 is effected manually by means of a lever or armv 29 attached to pivot plate 30 and is locked into the desired position by a locking pin not shown. cal oven closure 31 tits over or encloses rotatable molding cylinder 19 ina 'fixed or stationary position. Around oven 31, at each end thereof, is provided electrical heating means 32. These electrical heaters 32 are insulated and lagged. Oven 31 is attached to base plate 37 by which is beveled for fitting i-nto lip 22 of molding cylinder 19. This beveled end plate 33 is attached to a rotatable shaft 34 and passes freely through the end of oven 31. Shaft 34 is supported by thrust bearings 36 as shown. Thrust bearings 3,6.are attached by conventional means to support 37. The combination of plate 33, shaft 34 and The cylindri- Within oven 31 Vis end plate 33'V tion. Vmanifold adapted to eject a stream of air opposite to the thrust bearings 36 assists in maintaining dynamic balance of molding cylinder 19V during rotation. Support 37 is slidable upon track 33.1 One end of a pneumatic plunger 42 is connected by means of a boss member 43 to a table or stationary base support 44. The other end of plunger 42 is attachedto support 37 through a plunger shaft 41 and boss member 39. Support member 44 also supports pivot plate 30feeding unit 7, track 38 and track 51.

The cooling unit is indicated kby numeral 47 which comprises a water trough orreservoir 48 having a splash hood 49 which is slidable on track` 51. Numeral 52 indicates a handle for sliding the unit on track 51 in the directions as shown. A cross-sectional view in the direction 4 4 is diagrammatically illustrated in FIGURE 4. In addition to the above features, FIGURE 4 illustrates a wick 54 of leather, fabric or felt, for wetting the sides of molding drum 19 when it is in thefcooling posi- Numeral 53 indicates a perforated air conduit or direction of rotation of drum 19 during cooling. The air stream aids in the evaporation of the water and increases the rate of cooling, and also helps prevent splattering of water during rotation.

In the operation of the molding apparatus as shown in FIGURE l, a weighed quantity of a ilumine-containing thermoplastic in powder or granular form, such as a homopolymer of tritiuorochlor'oethylene having a ZST of about 235 (plasticized-with about 35 weight percent triiiuorocllloroethylene oil-ZST after plasticization about is placed in trough S while side 11 is positioned such that opening 12 is closed. The powdered polymer is distributed uniformly-along the-length -of trough 8. The molding cylinder19 is coated on the inside surface thereof with triuorochloroethylene telo'mer oil which vhas been heated Ito kabout 400 Ffto facilitate distribution. Coating the molding surface with plasticizer aids in maintaining the plasticizer content of the plastic constant during molding. Molding cylinder 19 is pivoted by arm 29 into axial alignment `with trough 8 and locked -into place. Cylinder 19 -is lthen rotated by motor 28 at about 800 to about 1500 rpm. The trough 8 is then inserted by sliding base 13 along track 15 such that the slot 12 of trough S is in alignment'with the width of molding surface 20. The sliding side 1:1 is withdrawn a predetermined amount to open sloty 12 and the trough vibrated by means of vibrator 14 until all of the powdered polymer is evenly and uniformly distributed over the entire cylindrical surface 20 of drum 19. Thereafter troughs is removed from the inside ofv drum 19 along track `15, and drum 19 while still rotating is then pivoted topand'locked in the molding position as shown in FIGUREI of the drawing.

'While drum .19 is still rotating at about 800 to about 1500 rpm., oven 31 which is continuously being heated is moved into place over drum 19 by means of plunger 42. Plunger 42 slides support plate 37 along track 38 until end plate 33 engages the ybeveled lip 22 of the open end of molding drum 19. The-plunger force-s plate 33 firmly against the end of drum 19 to securely seal the drum, and a predetermined pressure is maintained against the drum 19 and plate 33 by means of plunger 42 throughout the entire molding operation. Plate 33 and plunger 42 act as a relief valve to prevent the build-up ofexcess pressure in molding cylinder 19. Heating elements 32 raise the temperature of the oven to at least about 620 F., preferably about650 F. to labout- 660" F. The rotation of drum y19 is continued at 800 to 1500 r.p.m. for about one minute yand then is increased to between about 5000 and about 6000l rpm. or more.- .The centrifugal pressure should be at least 0.5 pounds per square inch on the polymer. The temperature is measured by means Vof thermocouples (not shown) Iattached to the Vinner surface of oven 31 below heaters 32 and by'a therm-ooouple attached to the inner surface in the middle unheated portion of oven 31. VWhen thertemperature of theloyen reachs'abo'ut 650 YF., the rotation is continued for about tive minutes or more. The over-all time for the heating operation and the molding operation is .about twenty to twenty-ve minutes, depending, however, on the polymer being molded. After about twenty-five minutes, heating elements 32 are turned off and the rotation of the drum is allowed to ldrop to about S to 1500 r.p.m. for cooling. Oven 31 is'retracted by means of plunger 42 to completely clear cylindrical drum 19.

While still rota-ting, cylinder 19 is pivoted on plate 30 by means of lever 29 to the position shown in relief in cooling unit 47 of FIGURE 1 and locked in place. In cooling unit 47, trough 48 contains Water and the wick 54 is pressed yagainst the side of rotating cylinder 19 by means of handle 52. Air is jetted through perforated conduit 53 to aid in the cooling of cylinder 19. Cylinder 19 is rapidly cooled to a temperature below 150 F., usually to approximately room temperature, in about ten minutes. After cooling, the rotation of cylinder 119 is stopped and the film or endless belt y23 is removed by slipping a knife or spatula under lip 22 of the open end of cylinder 19 to lift up the lm which -is then peeled yfrom the surface l20 of the cylinder 19 and removed from the inside of the cylinder.

The method or technique of molding of this invention can be used to make films, endless belts, gaskets, lockseals, lip-seals, etc., without departing from the scope of the invention.

Various conventional automatic mechanical and timing means may be used to provide fully automatic operation of the cycle without departing lfrom the scope of this invention.

The following examples are offered Ias a better understanding of the present invention and are not to be construed as unnecessarily limiting thereto.

EXAMPLE vI An endless belt of a thermoplastic polymer of triiluorochl-oroethylene was prepared with the apparatus of the drawing. The endless belt had a dimension of 1115A@ inches lay-flat x 9% inches wide x 10 mils thick. For preparing such a belt, a steel molding cylinder having a molding surface of 7.665 inches in diameter x 93/8 inches Wide was used. The inside of the molding drum was nickle-plated. A silicone resin was applied to the inside molding surface and heated to about 600 F. for about sixteen hours. The surface was then polished with a soft cloth to remove excess silicone resin. The molding surface was then coated with trifluorochloroethylene telomer oil plasticizer. The oven consisted of a steel tubing with an inside diameter 1/z-Iinch larger than the outside diameter of the molding cylinder. Two 20G-volt, Z300-watt heater bands positioned around the oven and insulated, as shown in FIGURE 1 of the drawing as numeral 32, supplied the exterior heat needed for molding. Three thermooouples were attached to the inside of the oven, one a-t each end of the cylinder adjacent the heaters and one in the middle section of the oven. These thermocouples were used to control the oven temperature. Water was used to cool the drum after molding.

About 811 grams of powdered polytrifluorochloroethylene plastic of a ZST of 235 (ZST after 35 weight percent plastioizerdlfo) having a maximum particle size of -about 150 microns was placed in feeding trough 8 of FIGURE 1. The feeding trough 8 was then inserted into the molding drum 19 whi'le the drum was rotating yat about 1000 r.p.m. Vibrator 14 was turned on and a uniform layer of powder was deposited on the inside surface of the molding cylinder. The above quantity of plastic had been 'determined as the amount required for making the belt of the dimensions previously stated.

After the feeding trough had emptied all of the powder on the inside surface of the molding cylinder, the feeding trough 8 was removed and the revolving drum 19 was ro-tated in the position shown in FIGURE 1 of the drawing. Oven 31 was inserted over the molding cylinder. The oven was warm from a previ-ous molding run. End plate 33 of the oven locked into place in beveled lip 22 of molding drum 19. The heat controls of the oven were set at 650 F. for the heater band at lone end of the oven By rotating the molding cylinder at the speed of about 1000 r.p.m., the powdered plastic is allowed to become thoroughly and uniformly heated to the conversion temperature of about 620 F. By increasing the speed to about 6000 r.p.m., the resin becomes homogeneous and is forced against the surface of the molding cylinder. Due to the high viscosity of the plastic at the molding temperature, the plastic does not flow in the molding cylinder.

Table 2 below shows the molding force at various revolutions per minute of the molding drum.

Table 2 Rpm Pressure, Gs

p.s.i.

After the above molding cycle had been completed, the oven 31 was removed from the molding cylinder 19 and the molding cylinder revolved into the cooling position where the air was rapidly cooled with water and the rotating speed dropped to about 800 r.p.m. An air blast accelerated the evaporation of water, increasing the cooling rate. After the molding cylinder had been cooled to room temperature, the rotation of the cylinder was stopped and the finished endless belt removed from the inside of the molding cylinder. The ZST of the finished belt was approximately 125.

When the polymer has reached the conversion temperature and the centrifugal pressure applied to the polymer is equal over the entire surface of the belt, a condition results in which the polymer can form itself to the contour of the molding surface while at the conversion temperature. Because the polymer does not flow during molding or cooling, no orientation stresses are built up in the final film. The result is that the belt or film has excellent dimensional stability upon aging. Under cross-polarized light no visible stresses were apparent. As compared to extruded film, the polytriuorochloroethylene plastic iilm prepared as above had a considerably higher ZST value. For example, a similar extruded film using a polytrifluorochloroethylene polymer, ZST 235, containing 25 weight percent telomer plasticizer had a ZST of only 105. Both ejection molding and extrusion confine the resin during conversion, thus preventing the escape of gases from the plastic. In centrifugal molding, gases are permitted to escape and the final product is therefore free from bubbles or voids.

EXAMPLE II An endless belt Was prepared in a manner similar to Example I from extrusion-grade Teon (a copolymer of tetrafluoroethylene and a small amount of peruoropropene) having a maximum particle size of 300 microns. The Vdrum 19 was loaded with 80 grams of this copolymer. The heat controls of the oven were set at about 700 F. for the heater band at one end of the oven and about 670 F. for the heater band at the other end of the oven. Otherwise the procedure was substantially the same as for Example I for time, rotation speed, etc. The top rotation speed of the molding drum was about 5900 r.p.m. A smooth uniform flexible belt was successfully produced by the above method.

EXAMPLE III Another run was made on Telon homopolymer .to produce an .endless belt in a manner similar to Example I. For this run on Teflon homopolymer (homopolymer of'tertauoroethylene) the controllers for the heater bands Were seit at 800 F. and 790 F., respectively. The moldingfdrum 19 was charged with 60 gramsof tetrafluoroethylene homopolymer screened to 420 microns as the largest particle size. After loading the drum with powdered homopolymer, the drum was rotated at about- 6000 rpm. to pack the powder. The drum was then slowed to about 1500 for about l minutes and then speeded up to a maximum speed of 6000 rpm. and held for about 40 minutes. A tough, flexible, Well-knit vendless belt Was produced With the homopolymer of tetrailuoroethyleine.

The present technique works exceptionally well for producing films of `fluorine-containing polymers Vof the examples which have la viscosity greater than 100,000 poises ait the molding temperature. This viscosity includes the Viscosity decrease as a result of plasticizers. Various modiications of time, rotation speeds and other operating techniques may become apparent to those skilled in the art without departing from lthe scope of this invention.

Having described my invention, I claim:

l. Molding apparatus which com-prises in combination (a) a pivotable base plate, (b) a motor attached to said pivotable base plate, (c) ahollow cylindrical molding drum having one closed end and one open end, (d) a rotatable shaft axially connecting said molding drum and said motor, (e) an olven having one closed end and one open end and of such size t-o fit over said molding drum, (f) a rotatable end plate adapted to lit contiguously with said open end of said molding drum and positioned inside of and attached to the closed end of said oven, (g) a track axially aligned with said molding drum, (h) means for moving said'oven along said track to pass over said molding drumand engage saiclendV plate'with the open end of said molding drum to form an enclosed mold-ing cylinder, (i) a triangular cross-sectional trough having an adjustable elongated opening in the bottomithereof and adapted to be insented within said hollow molding drum, (i) automatic vibrating means, attached to one end of said trough, (k) means for moving said trough inside of said` molding drumV when saidnaotor and molding drum are pivoted on said pivotable .base plate to an axially aligned position with said trough, (l) and means for cooling said molding drum.

2.Molding apparatus which comprises in combinationl (a) a pivotable base plate, (b) a motor attached to said pivotable base plate, (c) a hollow molding drum having one closed end iand one open end, (d) a rotatable shaft axially connecting said molding drum and said motor, (e) an oven having one closed end and open end and off such size to iit over said molding drum, (f) a rotatable end plate adapted to fit contiguously with said open end of said molding drum and positioned inside of and attached to the closedend of said oven, (g) means for moving said oven over said molding drum and engaging said yend plate with the open end of `said molding drum to form an enclosed molding cylinder, (h) a feeding trough having anelongated opening in the `bottom thereof and adapted tto =beinsertedwithin said hollow molding drum, (i) means for moving saidtrough inside of said molding drum when said motor and molding vdiru'm are pivoted on said pivotable base plate to an axially aligned position With said trough, (j) andrmeans for cooling said molding drum.

3. Molding apparatus which comprises in combination (a) a pivotable base plate, (b) a motor attached lto said pivotable base plate, (c) a hollow molding drum having one closed end and :one openend, (d) a rotatable shaft axially connecting said moldingdirum and said motor, (e) an oven having one closed end and one open end and of such size to t over said molding drum, (f) a rotatable end plate adapted to fit contiguously with said open `end of saidmolding drum and positioned inside of and attached to the lclosed end of said oven, and (g) means for moving said oven over said molding `drum and engage said end plate with the open end of said molding drum to form an enclosed molding cylinder. i References Sited by the Examiner UNITED STATES PATENTS 2,278,858 4/ 42 Fields Y18,--26 2,573,693 11/51 DeBell Q 18-26 3,077,635 2/ 63 Ringdal ,1S-26 FOREIGN PATENTS v ,45 3 2/ 52 Australia.,

MICHAEL V. BRINDISI, Primary Examiner.V

Claims (1)

1. MOLDING APPARATUS WHICH COMPRISES IN COMBINATION (A) A PIVOTABLE BASE PLATE, (B) A MOTOR ATTACHED TO SAID PIVOTABLE BASE PLATE, (C) A HOLLOW CYLINDRICAL MOLDING DRUM HAVING ONE CLOSED END AND ONE OPEN END, (D) A ROTATABLE SHAFT AXIALLY CONNECTING SAID MOLDING DRUM AND SAID MOTOR, (E) AN OVEN HAVING ONE CLOSED END AND ONE OPEN END AND OF SUCH SIZE TO FIT OVER SAID MOLDING DRUM, (F) A ROTATABLE END PLATE ADAPTED TO FIT CONTIGUOUSLY WITH SAID OPEN END OF SAID MOLDING DRUM AND POSITIONED INSIDE OF AND ATTACHED TO THE CLOSED END OF SAID OVEN, (G) A TRACK AXIALLY ALIGNED WITH SAID MOLDING DRUM, (H) MEANS FOR MOVING SAID OVEN ALONG SAID TRACK TO PASS OVER SAID MOLDING DRUM AND ENGAGE WITH END PLATE WITH THE OPEN

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